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From Steel to Tube
An overview of welded tube manufacture (continued) |
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Making The Pipe |
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The coils are received at the
Electric Weld (EW) Mill from the slitting line in widths which are governed
by the wall thickness and the outside diameter of the finished pipe. The
diagram above shows the various stages through the EW Mill from the coil to
the welded pipe. The coil is first uncoiled and fed through a strip
leveller, which flattens the strip and prepares it for forming. The front
and back of each coil is sheared by the strip end shears to present a clean
square edge to the flash welder. At the flash welder, the ends of the coil
are joined by flash butt welding and the upset cleaned off. This allows the
mill to run continuously. The flash welded section is cut out at a later
stage and discarded. The looping pits form a strip accumulator between the
flash welder and the Forming Mill which allows the EW Mill to continue
welding, and producing pipes while the flash welding operation between coils
is taking place.
On EW Mills which produce quality welded pipes, it is essential to
present consistent edge conditions to the mill to ensure that optimum
welding conditions can be maintained and this is achieved using the Edge
Scarfing Unit. The Edge Scarfing Unit removes the shear plain edge and
presents a square machine edged surface to the mill. The forming mill
progressively shapes the strip from the flat ingoing material to the closed
oval. Careful design of this section avoids the creation of unnecessary
residual stresses within the final product. |
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Regular checks are made
on mill alignment and precise setting of the mill is carried out to ensure
that the strip travels up the centre of the roll train so that on reaching
the end of the forming mill, there is no tendency to twist thus avoiding
uneven working of strip edges. The last stands within the forming mill are
generally referred to as the fin passes which work the edge of the strip to
present a consistent edge profile to the welding vee. The forming mill is
adjusted to standard settings and the actual values recorded for each size
and gauge. Discipline in the control of the mill set up contributes to the
assurance of consistent product quality and weld integrity.
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The strip edges are
heated to a welding temperature by the high frequency welder. The welder is
a large radio frequency oscillator producing and alternating current of
200.400 kHz which resistance heats the strip edges. Due to the two phenomena
associated with radio frequency electric current (i.e. The skin effect and
the proximity effect) this current concentrates in the surface of the strip
edge.
At the weld head the two heated edges are brought together and
pressure applied to form a forged weld. All previously liquid metal is
expelled together with any oxides and the plastic areas behind the heated
edges upset. The geometry of the weld area is very important and the ingoing
and outgoing circumferences are measured to assess the amount of metal,
which has been pushed out. The symmetry and dimensions of the heat pattern
are regularly checked by cutting a sample from the tube, polishing and
etching a cross section of the weld and examining the microstructure under a
microscope.
Subsequent heat treatment completely removes the heat pattern
resulting in a uniform structure in the weld region. After welding,
the internal and external weld flash or bead is planed from the tube. The
strip edges are thickened in the fin passes before welding which allows the
internal and external bead planning equipment to marginally cut into the
parent metal without reducing the tube
wall below the nominal thickness, (Figure 14). The weld bead is regularly
checked for uniformity and integrity.
The weld line is then water cooled to lower the temperature of the
pipe as it enters the sizing mill. The sizing mill rounds up the pipe and
marginally reduces its diameter to give the required finished dimensions.
Within the sizing mill the weld line is inspected continuously using
ultrasonic shear wave and surface wave techniques together with eddy current
testing. The sizing mill is also used to produce a straight pipe by
adjusting the final restraining roll pass at the end of the sizing section.
The rotary cut-off is used to cut the continuous pipe into the required
lengths unless stretch reduction is required when the pipes are left in
lengths of up to 122 metres. |
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